1. Field of the Invention
The present invention relates, in general, to brushless DC motors and, more particularly, to a brushless DC motor with a rotor, the rotor being coupled to a hydrodynamically supported shaft and being free from any axial vibrations during a high speed operation of the motor, thus effectively reducing operational vibrations and noises.
2. Description of the Prior Art
As well known to those skilled in the art, small-sized precision motors, typically used in office machines, are required to be designed to rotate at high speeds and provide dynamic characteristics of low operational vibrations and noises in order to meet the necessity of high speed operation and provide a large capacity of such office machines. Therefore, it is a recent trend to change bearings for such motors from ball bearings into hydrosintered or hydrodynamic bearings with excellent dynamic characteristics.
FIG. 1 is a sectional view of a spindle motor using a conventional hydrodynamic bearing. As shown in the drawing, a sleeve 1a is vertically and concentrically arranged on the base panel of a motor housing 1 through a fitting process, while a shaft 2 is rotatably and downwardly inserted into the sleeve 1a. A core 1b, with a coil 1c, is arranged around the sleeve 1a, thus forming a stator of the motor. The top end of the shaft 2 is coupled to a cap-shaped rotor 3, thus being rotatable along with the rotor 3. A cylindrical magnet 3a is attached to the inner surface of the rotor's sidewall, thus surrounding the core 1b. When the motor is started, electric power is applied to the coil 1c of the core 1b, thus allowing the magnet 3a to generate magnetic force. The rotor 3 is thus rotated along with the shaft 2 at high speeds.
In the above spindle motor, the hydrodynamic bearing comprises a plurality of dynamic pressure grooves 2a, which are formed on the shaft 2 and are filled with oil. When the shaft 2 is rotated inside the sleeve 1a at high speeds, oil, filled in the gap between the sleeve 1a and the shaft 2, generates a hydrodynamic pressure at the grooves 2a and effectively supports the shaft 2 in a radial direction during a high speed rotation of the shaft 2. When the shaft 2 is rotated at high speeds as described above, the rotor 3, carrying a disc (not shown) thereon, is rotated at high speeds, thus allowing data stored on the disc to be reproduced.
However, such a conventional hydrodynamic bearing for spindle motors is problematic in that it fails to support the shaft 2 in an axial direction and only supports the shaft 2 in a radial direction. Therefore, another bearing has to be provided in the motor for supporting the shaft 2 in an axial direction even though it is very difficult to produce the radial support hydrodynamic bearing and the axial support bearing and precisely set the two bearings in the motor.
In an effort to overcome the above problems experienced in the above spindle motors, Japanese Patent Laid-open Publication No. Hei. 7-110,028 and Japanese Laid-open Publication No. Sho. 56-20,828 individually disclose a technology of limiting axial vibrations of a rotor in motors. In each of the above Japanese patents, the shaft of a motor is axially supported by a washer-type structure capable of forming a hydrodynamic pressure on the shaft in an axial direction using elastic liquid. However, the above Japanese technologies have a problem in that such a washer-type structure complicates the production process of motors and remarkably increases production cost, thus being less likely to be produced in commercial quantity. Another problem of the above Japanese technologies is that the washer-type structure only provides low axial dynamic characteristics for the shaft, thus failing to radically remove axial vibrations of the shaft and rotor.